Khalid Said

Porous silicon in crystalline silicon solar cells: A review and the effect on the internal quantum efficiency

Crystalline silicon (c-Si) is the dominant semiconductor material in use for terrestrial photovoltaic cells and a clear tendency towards thinner, active cell structures and simplified processing schemes is observable within contemporary c-Si photovoltaic research. The potential applications of porous silicon and related benefits are reviewed. Specific attention is given to the different porous silicon formation processes, the use of this porous material as anti-reflection coating in simplified processing schemes and for simple selective emitter processes and its light trapping and surface passivating capabilities, which are required for advantageous use in thin active cell structures. Our analysis of internal quantum efficiency data obtained on both conventional and thin-film c-Si solar cells has been performed with the aim of describing the light diffusing behaviour of porous Si as well as investigating the surface passivating capabilities. An effective entrance angle of 60° is derived, which corresponds to totally diffuse isotropic light, and the importance of a correction for absorption losses in the porous layer is illustrated. Furthermore, photoconductivity decay measurements of freshly etched porous Si on float-zone p-type Si indicate a strong bias-light dependency and a fast degradation of the surface recombination velocity.

Design, fabrication, and analysis of crystalline Si-SiGe heterostructure thin-film solar cells

One possible method to improve the efficiency of crystalline silicon (Si) solar cells is by alloying with germanium (Ge). Although the improved absorption of the alloy leads to a gain in the current, the reduction of the alloy bandgap causes a loss in voltage, which overrides the increased current of the SiGe-alloy solar cell. There has been a number of theoretical studies to circumvent this behavior. However, to date there has been no detailed study, which discusses the technological implementation of these concepts in solar cells. In this paper, the design issues of crystalline Si-SiGe heterostructure will be dealt with in an attempt to reduce the effect of the increased dark current of the alloyed cells, while at the same time sustaining the enhancement in the current. The enhanced back surface field at the back p/sup +/-Si/p-SiGe interface reduces the base component of the recombination current of the heterostructure cell if recombination caused by dislocations is neglected. A higher infrared (IR) response which results in a higher short-circuit current (2 mA/cm/sup 2/ higher than a reference Si cell) has been recorded for the Si-Si/sub 0.9/Ge/sub 0.1/-thin-film structure of 15 /spl mu/m thickness. The reduction in dark saturation current which has been predicted based on the theoretical calculations could not be realized in the heterostructure SiGe/Si cell due to the degradation effect of the misfit dislocations that decreases the bulk lifetime, and increases the interface recombination velocity. In a structure which contains a p/sup +/-SiGe buffer layer, an efficiency of 12.5% is achieved for a SiGe cell with 15 /spl mu/m thickness without texturing or optical confinement, which is about the same as the Si reference cell with equal active thickness, but with a higher short-circuit current. These results, for the first time, experimentally prove that alloying with Ge offers a higher current and might have a room for improving the efficiency of the multijunction solar cells or dual bandgap cells when SiGe is used to convert the IR-part of the spectrum.

High quality, relaxed SiGe epitaxial layers for solar cell application

Abstract Epitaxially grown, relaxed Si 1− x Ge x layers with x≤0.1 were grown on a Si (100) substrate by means of reduced pressure chemical vapor deposition at a temperature of 750 or 800°C. The analysis carried out on the grown layers revealed a very high material quality indicated by the low density of dislocations (10 5 cm −2 ) and the high diffusion length which was deduced from the measurements of electron beam induced current (EBIC) performed on the as-grown layers. Transmission electron microscopy (TEM) measurements showed that the threading dislocation segments do not extend inside the layer but are rather confined to the Si/SiGe interface, which results in a low density of dislocations in the material. The processed solar cells made from these SiGe layers showed a higher infrared response than those made of a corresponding Si grown and processed under similar conditions. No degradation of the solar cell performance caused by the dislocations in the SiGe layers has been observed.

CRYSTALLINE SILICON THIN FILMS : A PROMISING APPROACH FOR PHOTOVOLTAICS?

In this paper we review the achievements in the field of silicon crystalline thin film solar cells and correlate these with the different types of growth techniques and substrates. As a starting point we discuss the characteristics of photovoltaic devices based on the use of highly doped monocrystalline substrates as mechanical carriers for the thin films. These films are epitaxially deposited from the gas (CVD) or liquid phase (LPE). The comparison of both techniques is extended to growth on defective silicon substrates, i.e., multicrystalline wafers or silicon ribbons. The intrinsic grain boundary recombination activity in the thin films is assessed as a function of the deposition technique. Bulk passivation by hydrogenation considerably improves the recombination properties. The optimization of the hydrogen passivation conditions is looked at in conjunction with the used surface passivation process. This review is completed with the approaches to realize thin film cells on nonsilicon substrates, including recrystallization in solid and liquid phases.

Interaction between bulk and surface passivation mechanisms in thin film solar cells on defected silicon substrates

Common features of thin film silicon (TFSi-) solar cells hydrogenated in a microwave induced remote plasma (MIRP) are studied. The thin films were epitaxially grown on ribbons and multicrystalline silicon (mc-Si). Optimal hydrogenation renditions are presented in relation to the sample conditions such as the surface passivation, the active carrier concentration in the epitaxial layer and the substrate material. The MIRP-hydrogenation is studied in the temperature range of 350-415/spl deg/C. A high passivation efficiency is observed at 400/spl deg/C for all substrates in spite of an unfavourable hydrogenation regime at 375/spl deg/C. In the absence of an oxide during the hydrogenation, an optimal hydrogenation time of 1 hour is observed irrespective of the substrate used. This optimal hydrogenation time is a consequence of the interaction between the incoming atomic hydrogen and the boron dopant in the epitaxial layer.

SiGe Thin-Film Structures for Solar Cells

In order to study their applicability as the active base material in Si thin crystalline film solar cell technology, SiGe relaxed layers grown by Liquid Phase Epitaxy (LPE) and Chemical Vapor Deposition (CVD) on Si substrates are investigated by optical and electrical measurements (TEM, EXD, PL, EBIC). The main results of this work is to point out the improvement of the SiGe active base layer by using smooth Ge graded SiGe buffer layer and remote plasma hydrogenation. TEM, EXD, PL experiments show the effect of the Ge graded buffer layer grown using LPE, by confining the threading dislocations in the SiGe buffer layer close to the Si/SiGe interface. EBIC measurements reveal low recombination activity of dislocations at 300 K providing the diffusion length exceeds the 15 {micro}m layer thickness. The enhanced luminescence of SiGe near bandgap indicates that remote plasma hydrogenation induces a decrease of the non-radiative recombination pathways due to dislocations on CVD layers where defect recombinations dominate as indicated by EBIC measurements. This study points out the importance of controlling relaxed SiGe layers with good minority carrier recombination quality as a key issue for the optimization of new SiGe/Si based solar cells.

SiGe layer structures for solar cell application grown by liquid phase epitaxy

Si/sub 1-x/Ge/sub x/ layers with x=0.09-0.27 were grown on Si (111) substrates by liquid phase epitaxy (LPE). The formation of dislocations caused by the lattice mismatch was reduced from 5/spl times/10/sup 7/ to 7/spl times/10/sup 5/ cm/sup -2/ by growing buffer layers in which the Ge content increased continuously from 4 at% Ge at the substrate/buffer interface up to the Ge content of the Si/sub 1-x/Ge/sub x/ base layer. The p-type SiGe layers grown from indium solution are about 20 /spl mu/m thick. The carrier concentration of 5/spl times/10/sup 16/-2/spl times/10/sup 17/ cm/sup -3/ depends on Ge content and Ga doping. With a thin-film cell processed from a 14 /spl mu/m thick LPE-grown Si/sub 0.9/Ge/sub 0.1/ base layer, an efficiency of 9.1% was achieved even without the presence of a buffer layer.

Electrical properties of SiGe layers grown by LPE and CVD

In this paper we present electrical properties of a few microns thick, relaxed SiGe layers for solar cell application. The layers were characterized by means of electron beam induced current (EBIC), X-ray microanalysis (EDX), backscattered electrons, and Hall effect analysis.

Advanced concepts and options for thin-film crystalline Si solar cells

The purpose of this paper is twofold. Firstly, it presents a concise overview of the benefits to be expected from the development of a thin-film crystalline Si solar cell technology on a low-cost substrate as well as the technical and physical challenges encountered. It becomes clear that a large number of options are open in terms of substrate, deposition technology and solar cell technology. Secondly, focus is placed on the approach chosen by the thin-film crystalline Si solar cell team of IMEC and the results and progress of understanding achieved by the group over the last five years. Basically, their technical strategy is based on an evolutionary scheme for the cell design. More specifically, a transition is made from a classical two-side contacted cell design to a one-side contacted cell type which paves the way for a monolithic module design. For the technical realisation of the thin crystalline Si layers, chemical vapour deposition is expected to be the preferred technique, at least in a short and mid-term perspective.

Comparison of bulk and surface passivation properties of plasma nitrides on Si and SiGe solar cells

In this paper the effects of direct and remote plasma nitrides on the performance of Cz-Si and SiGe bulk cells are analysed. The surface passivation properties of both kinds of nitrides are compared to a thin thermal oxide, grown at high temperatures. Internal quantum efficiency measurements prove that the surface recombination velocities are lowest in case of the remote plasma nitride layer. The extracted values of the surface recombination velocity are as low as 1.5/spl times/10/sup 3/ cm/s for the remote plasma nitride, whereas the value for the thermal oxide is twice as high, comparable to the value obtained for the direct plasma nitride. SiGe-cells show the same tendency, although the blue response is lower in absolute value. However, when using the appropriate SiGe-absorption coefficient for parameter extraction by PC-ID, the surface recombination velocity of the plasma nitrides on SiGe-emitters is comparable to what is found in case of Si. In addition, we also found that for both remote plasma and direct nitride layers, there is a significant enhancement of the red response of all types of cells, compared to the samples without nitride. A comparison was made of the behaviour of the cells, with starting lifetime of 10-30 /spl mu/s, showing that the enhancement of the red response by the use of a plasma nitride is comparable to the beneficial effects of a remote plasma hydrogenation. However, for starting lifetimes lower than 5 /spl mu/s, the separate hydrogenation step brings an additional improvement of the red response compared to the only-nitride case and the effect of the two treatments seems to be cumulative.